Series equipment is sometimes connected between a central office (CO) line and terminal equipment such as PABXs to provide specialized services, such as call redirection, data modification, automatic recall, etc. Typically the terminal equipment is connected either in series with the CO line, or is connected in series with the series equipment to the CO line. In the first case, the series equipment must have a hold circuit to hold the CO line "off-hook" while the series equipment is in series with the terminal equipment line and the CO line. In the second case the series equipment must include a battery feed to supply a hold circuit in the terminal equipment while the series equipment is in series with the terminal equipment line and the CO line.
The hold circuit typically consumes up to three watts of power, and is required to meet stringent overvoltage safety requirements. The battery feed supplies the internal terminal equipment hold circuit with direct current, and dissipates power and has strict noise requirements. These circuits have been found to dissipate the majority of the power used by the series equipment.
The dissipation of power, the meeting of overvoltage requirements and the meeting of noise requirements are all undesirably expensive.
FIG. 1 is a schematic diagram of series equipment in accordance with the prior art. Terminal equipment 1 is connected via tip and ring leads of a terminal equipment line 3 to tip and ring leads 5 of a central office (CO) line, through double pole double throw contacts 7 of a relay. Typically a protective device 9 such as a varactor is connected across the CO line.
When the contacts 7 are in one position, the terminal equipment is connected directly (cut through) to the CO line, and therefore the direct current supplied via the CO line is applied to the terminal equipment, to supply the internal terminal equipment hold circuit, for example. When the contacts 7 are in another position, the cut through connection is severed, and the terminal equipment is connected to the series equipment 11.
The connection of the terminal equipment through contacts 7 in the latter position is via DC blocking capacitor 13 and bidirectional amplifiers 15 to signal processing circuits 17, such as audio paths, a modem, a dual tone multifrequency (DTMF) transceiver, etc., which can be controlled by a microprocessor 19. The signal processing circuits 17 are also connected via bidirectional amplifiers 21, impedance matching circuit 23 and capacitor 25 to one winding of transformer 27.
The other winding of the transformer is connected via DC blocking capacitor 29 to the tip and ring leads of the CO line 5.
Due to there being DC blocking elements in series with the CO line 5 and the terminal equipment line 3, the internal hold circuit in the terminal equipment cannot be used to hold the CO line. Therefore a separate hold circuit 31 is connected across the CO line 5. Another contact 33 is connected in series with the hold circuit and the connection of the transformer 27, and the CO line.
Also due to the aforenoted DC blocking elements, current from the CO line cannot be used to feed the terminal equipment line 3. For that reason a battery supply circuit 35 is connected across the terminal equipment line, via contacts 7.
In operation, with the contacts 7 in one (upper) switched position, and with contact 33 open, the terminal equipment line 3 is cut through to the CO line 5. DC is supplied from the CO line to the terminal equipment, and the internal hold circuit in the terminal equipment can hold the CO line offhook.
With the contacts 7 in the other (lower) switched position, and with contact 33 closed, a DC path from the CO line 5 to the terminal equipment line 3 is blocked, but there is an AC path between the lines via the series equipment 11. Due to the blocked DC path, DC current is supplied from battery supply circuit 35, and separate hold circuit 31 is connected to the CO line 5.